Merge pull request #4454 from esl/fix-migration-guide

Fix the description of log handler migration

Author: jacekwegr

doc/migrations/6.3.0_6.3.1.md

@@ -1,6 +1,7 @@
 ## Log Handler Configuration
 
-The shell log handler in `app.config` should be named either "default" to enable shell logs or "none" to suppress them.
+The shell log handler in `app.config` should be renamed from `shell_log` to `default`.
+Otherwise, there would be duplicate logs in the shell because of the [lager removal](https://github.com/esl/MongooseIM/pull/4393).
 
 ## CA Certificate Configuration